Thanks to Robert for submitting the web site information!
The Services and Facilities page notes that there is a drop box for USPS, FedEx and UPS adjacent to the Information Desk.
The Information Desk is pre-security in the hallway between the Central Hall (main entrance) and Baggage Claim.
The Services page also notes that you can buy postage at the Information Desk if you’re returning items that won’t make it past security. They may also carry regular stamps (but this is not confirmed).
Last updated 19 August 2018